Tetraethyl orthosilicate is a major chemical compound with a formula Si(OC2H5)4. The compound is frequently abbreviated as TEOS and consists of four ethyl groups attached to an SiO4 ion that is referred to as an ortho silicate. TEOS can also be considered to be the ethyl ester of orthosilicic acid, Si(OH)4 and is a prototypical alkoxide. TEOS is a tetrahedral molecule with many analogues most of which are prepared by alcoholysis of silicon tetrachloride.
Typically TEOS is hydrolyzed and condensed with mineral acid catalysts such as hydrochloric acid or sulfuric acid. TEOS is mixed with sufficient alcohol to allow its reactant water to be partially miscible in the presence of an acid catalyst. This allows the initial reaction to take place where one of the ethoxy groups is replaced by a water molecule liberating ethanol as a by-product. This is illustrated by the following diagram:Si(OC2H5)4+H2O→(C2H5O)3SiOH+C2H5OH
The condensation of silanol groups derived from the hydrolysis reaction is a competing reaction in the presence of acid catalyst. This is illustrated by the following diagram:(C2H5O)3SiOH+HOSi(OC2H5)3→(C2H5O)3SiOSi(OC2H5)3+H2O
Thus the polymerization of TEOS with water in the presence of a catalyst results from sequential hydrolysis of ethoxy groups and condensation of silanol groups in this over simplified view. In fact, there are other reactions, such as condensation of silanol with ethoxy groups that provide equivalent polymerization. All these processes result in polymers of increasing complexity, variety in physical properties, and usefulness in their intended applications.
Typically these polymers are thought of as combinations of linear, cyclic and polycyclic polymers. Physical properties such as viscosity, viscosity stability, and available silica (SiO2 wt %), are dependent on the amount of water reacted with the TEOS. This is expressed as a percentage of the theoretical amount of water to replace all the ethoxy groups bound to silicon. The stoichiometric amount of water is 2 moles of water to 1 mole of TEOS. This is referred to as 100% hydrolysis. When reacted with this amount of water, the resulting polymer has the physical properties of amorphous silica. This can be observed by reacting TEOS with 2 moles of water and then removing the by product ethanol by distillation. What remains is a solid material with a high percentage of silica remaining in the composition, typically 98% or higher. In practice it has been found that in order to obtain usable physical properties, a flowable liquid, only 40% of the theoretical water is added and then the by-product ethanol is removed. This results in a low viscosity liquid that contains 40% available SiO2 by weight. This material has become an item of commerce, known as Silbond 40 or Dynasil 40.
Tetraethyl orthosilicate also has many applications because of its easy conversion into silica. For example, it may be used for chemical mechanical polishing as outlined in a Negrych et al. U.S. Pat. No. 6,334,880 that is assigned to Silbond Corporation, the same Assignee as the present application. Another application relates to the synthesis of trimethylsiloxypolysilicates (MQ Resins) that are useful as additives for release coatings in pressure sensitive adhesive applications and liquid silicone rubber. The role of MQ Resins in such formulations is to modify the properties of the cured silicone rubber. The silicone rubber is hardened by this component to a higher modulus. For pressure sensitive release applications this increases the release forces of the adhesive. In both applications, the presence of the low molecular weight MQ Resins is detrimental.
A further application for TEOS where polymer distribution with reduced amounts of low molecular weight material are desired relates to the formation of polysilicate binders for zinc rich coatings, investment casting, refractory, sand core, and ceramic articles in general. These applications are well know in the prior art for ethyl polysilicates. A developing limitation for all these applications is the amount of low boiling alcohol by-product that is liberated during the further hydrolysis of ethylpolysilicate. This raises the amount of volatile organic components (VOC) present in the user's formulation. More environmentally friendly compositions that still meet the requirements of the user are desired. For example, 40% hydrolyzed ethylpolysilicate releases 2.4 moles of ethanol per equivalent of polymer, a restricted VOC component, while a 70% hydrolyzed ethylpolysilicate releases only 1.2 moles of ethanol per equivalent of polymer, a 50% reduction.